Conventionally, the tissue harmonic imaging (THI) method has been widely known as a method for obtaining a B-mode image with a higher spatial resolution than that obtained in usual B-mode imaging. The THI is an imaging method using a nonlinear component (e.g., a harmonic component such as a second-order harmonic component) included in a reception signal.
In the THI method, various types of signal processing are performed such as a phase modulation (PM) method, an amplitude modulation (AM) method, and an AM-PM method in which the AM method and the PM method are combined together, for example. In the PM method, the ultrasonic wave with inverted phases having an identical amplitude is transmitted twice on each scanning line and the resulting two reception signals are added up. Through the addition processing, a signal is obtained in which a fundamental wave component is cancelled and the second-order harmonic component generated by a second-order nonlinear phenomenon mainly remains. In the PM method, this signal is used for imaging the second-order harmonic component to obtain an image.
The THI method has been put into use in which imaging is performed by using a harmonic component having a broad bandwidth including the second-order harmonic component and a difference frequency component in the reception signal. In an imaging method utilizing the difference frequency component, the ultrasonic wave in which two frequency waves are combined, that is, a composite waveform in which two fundamental waves with different center frequencies are combined, is transmitted twice on each scanning line with inverted phases. The resulting two reception signals are then combined. The composite signal is a composite harmonic signal including the second-order harmonic component on the lower frequency side and the difference frequency component generated by a second-order nonlinear phenomenon. The composite signal is the harmonic echo having a broader bandwidth than the signal obtained in the above-described THI method.
The harmonic component generated by the second-order nonlinear phenomenon includes a low-frequency component mainly including a direct current (DC) in addition to the harmonic component targeted for imaging (e.g., the second-order harmonic component). The low-frequency component is also called a zeroth-order harmonic component or a DC harmonic component. If the transmission ultrasonic wave has a broad bandwidth, for example, the zeroth-order harmonic component may overlap with the second-order harmonic component. Alternatively, if the transmission ultrasonic wave has a broad bandwidth, for example, the zeroth-order harmonic component may overlap with the difference frequency component.
In that case, the center frequency becomes lowered as depth (distance) from the transmission position is increased due to the attenuation of the frequency dependence. This causes the zeroth-order harmonic component to affect at a non-negligible level in a deep part, leading to deterioration in the image resolution at a deep part. If the zeroth-order harmonic component is reduced through filtering processing, however, the second-order harmonic component on the lower side or the difference frequency component on the lower side are also reduced. This generates an uneven image in the depth direction due to insufficient penetration.